So-called “transparent screen” technology, in which a transparent screen displays images, has been well-known for some time. However, a screen that can ensure visual clarity through the screen (in other words, an image on the other side of the screen can be recognized as an image) while also ensuring a wide viewing angle and quality for images displayed on the screen does not currently exist. Thus, there are currently no transmissive or reflective projection display devices that include a high quality “transparent screen.”
The reasons for this will be described below.
In order to generate images on a screen, it is necessary to scatter light on the screen. Additionally, in order to ensure a wide viewing angle, it is necessary to increase the degree of light-scattering to a certain extent. When light is scattered, however, visual clarity through the screen declines.
This is due to the fact that it is difficult to both a) scatter light while ensuring image quality and a wide viewing angle, and b) ensure visual clarity through the screen, since there is a fundamental trade-off that exists between these two concepts.
Conventionally, there have been many transparent screens developed that included a scattering layer in which a scattering agent was spread across a transparent supporting substrate. Since the scattering layer by itself does not scatter enough light, transparent screens that further included a diffraction member (diffraction sheet) such as a holographic film were developed.
FIG. 18 illustrates a schematic configuration of a conventional transmissive projection display device 100.
As shown in FIG. 18, the conventional transmissive projection display device 100 includes: a screen 101; and a projector 104 that projects projector light onto the screen 101.
In addition, the screen 101 includes a diffraction member 102 and a scattering member (scattering sheet) 103.
The diffraction member 102 diffracts light so that projector light from the projector 104 can be efficiently oriented in the front direction of the screen 101. Since diffraction selectivity can be set in regards to the wavelength and angle of incidence of light, it is possible to selectively diffract only projector light while not diffracting any ambient light.
While it is possible to orient light towards the front by diffraction alone, diffraction by itself is not enough to disperse light to obtain a wide viewing angle (this ability is also known as a “scattering function”). Thus, by providing a scattering member 103, light is dispersed and a wide viewing angle can be ensured.
However, scattering members 103 included in conventional transmissive projection display devices 100 scatter both projector light from the projector 104 and ambient light, regardless of the polarization direction of the light. Thus, while a wide viewing angle can be obtained, it is not possible to ensure visual clarity through the screen 101, which is a problem.
In order to resolve such problems, Patent Document 1 discloses a holographic screen that includes a light-scattering element that only scatters light that enters from within a prescribed range of angles of incidence.
FIG. 19 illustrates a schematic configuration of a conventional holographic screen 201 disclosed in Patent Document 1.
As shown in FIG. 19, the holographic screen 201 includes: a holographic element 211 that has a function of diffracting projected light 221 that has been projected by a projection device 202; and a light scattering element 212. The light scattering element 212 is disposed on the projection device 202 side of the holographic element 211, which is the side of the holographic element 211 that is opposite of the viewer 206 side, with an adhesive layer 213 interposed therebetween.
By scattering incident light whose angle of incidence falls within a prescribed range, the light scattering element 212 can selectively scatter only projected light 221 projected by the projection device 202 and avoid scattering any ambient light.
Therefore, it is possible to obtain a wide viewing angle while at the same time ensuring visual clarity through the screen.
Patent Document 2 discloses a holographic screen that includes a directional scattering hologram that corresponds to the light scattering element 212 described in Patent Document 1.
Similar to the light scattering element 212 in Patent Document 1, it is necessary for the degree to which this directional scattering hologram scatters light to vary according to the angle of incidence of the incident light. It is therefore extremely difficult to industrially produce such a hologram and such a process is very expensive.
Patent Document 3 discloses a holographic screen in which the haze level is limited to a predetermined range.
Patent Document 4 discloses an alignment film that has selectivity with respect to the polarization direction of linearly polarized light, or in other words, has a transmission axis (a direction in which the least amount of scattering occurs) and a scattering axis (a direction in which the largest amount of scattering occurs) on the surface of the film.
In the screen disclosed in Patent Document 4, the alignment film and a polarizing element are stacked on each other, a dichroic polarizing plate is used as the polarizing element, and the alignment film and the polarizing plate are stacked such that the transmission axis of the alignment film matches the absorption axis of the dichroic polarizing plate. The scattering axis direction of the alignment film matches the polarization direction of light exiting from a liquid crystal projector.
Patent Document 4 discloses that, as a result of this configuration, it is possible to effectively scatter linearly polarized light that contributes to forming images and obtain a bright display while suppressing scattering of light in the transmission direction, which is not related to forming images, and also possible to absorb polarized light via the dichroic polarizing plate; thus, it is possible to display high contrast images.